As a hydrocarbon wellbore is being drilled, hydrocarbons from the formation mix with the drilling fluid and are produced to surface. Mud gas or fluid logging is the practice of removing some of those hydrocarbons from the drilling fluid and measuring their concentration and composition. This practice can provide operators with their first measurement of the hydrocarbons present in the subsurface. Mud gas logging is performed commercially today by Schlumberger and several other companies. The primary analytical tool used to evaluate the hydrocarbons is a gas chromatograph. This technology is effective, but it has the disadvantage that the measurement time can be relatively slow. For example, a single analysis of hydrocarbons requires approximately one minute. For better depth resolution, it would be highly desirable to perform the analysis in a much shorter amount of time.
In the use of gas chromatography, gases are separated using a column and detected with a flame ionization detector or, in the case of Schlumberger's fluid logging service, with a mass spectrometer. The analysis time of about 1 minute is mainly due to the time required to separate the components using the gas chromatograph. Two techniques to accelerate the measurement by replacing the gas chromatograph have been described in the literature.
First is a technique called direct quadrapole mass spectrometry (DQMS), produced by Fluid Inclusion Technologies Inc. and described in a presentation titled “Analysis of Borehole Gas with Direct Quadrupole Mass Spectrometry” presented at the 2010 AAPG Hedberg Conference in Vail, Colo. (AAPG Search and Discovery Article #90110, Jun. 8-11, 2010). This method can be applied on gases liberated by crushing cuttings. In this technique, gases are analyzed using mass spectrometry without prior separation (i.e. without a gas chromatograph). Gases are ionized using electron ionization at 70 eV, a technique that causes analytes to break up into ionized fragments (a hard ionization technique). The resulting fragmentation pattern of each gas of interest is unique, in principle allowing each gas of interest to be quantified. However, the fragmentation patterns of the different gases are extremely similar, especially among isomers, meaning that implementation often requires unrealistically high signal-to-noise ratios and does not provide a proper differentiation between isomers of interest.
Second is a technique called selected ion flow-tube mass spectrometry (SIFT-MS). This instrument is described in International Journal of Mass Spectrometry 268 (2007) 38-46. In this technique, gases are analyzed using mass spectrometry without prior separation (i.e. without a gas chromatograph). Mass spectrometry alone can resolve the different alkanes, but it cannot resolve isomers. To distinguish isomers, the analysis is repeated using different ionization schemes, particularly using chemical ionization with different chemicals, some of which preferentially ionize branched over linear alkanes. A disadvantage of the technique is that these ionization schemes are quite complex (they involve termolecular reactions), meaning the results may not be robust enough but instead depend sensitively on the presence of various impurities. Additionally, the sensitivity for various hydrocarbon species can vary by orders of magnitude, requiring the analyte to be concentrated or diluted prior to analysis.